Ninja is not proc based. It's pretty steady damage with an amount of burst damage. Ninja also has combos like Monk, but they just have a different feel. For Monks, your combo leads to a damage and skill speed self-buff. For Ninjas, your combos lead to a slashing resistant debuff or just direct damage.

Ninja is pretty fun once you get to level 40ish. Dragoon is pretty fun when you get to level 30ish. Dragoon for me feels like a Ret Pally if you are used to WoW, and Ninja is like a Rogue.

IMO:
Monk = Constant movement between flank and rear depending on ability you use.
Dragoon = lots of different buffs and debuffs on different cooldown timers to maintain.
Ninja = lots of different Ninjutsu to manage.

Ninjutsu is an interesting mechanic that provides either straight up damage, buffs, debuffs, or a bind. So you constantly have to think on the fly which Ninjutsu you want to use for different situations.